TheMetal - Insulator Transition of the Magnéli phase

– The metal-insulator transition (MIT) of the Magnéli phase V4O7 is studied by means of electronic structure calculations using the augmented spherical wave method. The calculations are based on density functional theory and the local density approximation. Changes of the electronic structure at the MIT are discussed in relation to the structural transformations occuring simultaneously. The analysis is based on a unified point of view of the crystal structures of all Magnéli phase compounds VnO2n−1 (3 ≤ n ≤ 9) as well as of VO2 and V2O3. This allows to group the electronic bands into states behaving similar to the dioxide or the sesquioxide. In addition, the relationship between the structural and electronic properties near the MIT of these oxides can be studied on an equal footing. For V4O7, a strong influence of metal-metal bonding across octahedral faces is found for states both parallel and perpendicular to the hexagonal c hex axis of V2O3. Furthermore, the structural changes at the MIT cause localization of those states, which mediate in-plane metal-metal bonding via octahedral edges. This band narrowing opens the way to an increased influence of electronic correlations, which are regarded as playing a key role for the MIT of V2O3. The vanadium oxides have been attracting a lot of interest for many years, in particular due to their metal-insulator transitions (MIT). Special focus has been on the prototypical compounds VO 2 and V 2 O 3 , which were studied quite extensively [1, 2, 3]. However, much dispute remains as concerns the origin of the phase transitions. While it is agreed that these arise from a delicate interplay of electron-phonon coupling and electronic correlations, the relative importance of these two mechanisms is still under controversal discussion. At a temperature of 340 K, VO 2 (3d 1) undergoes an MIT as well as a simultaneous structural transformation from the rutile to a monoclinic structure. This combined phase transition can be understood from electronic structure calculations, which point to a Peierls instability of the one-dimensional d (d x 2 −y 2) band in an embedding background of the remaining V 3d t 2g states [4]. Nevertheless, the optical band gap of the insulating phase was missed by the calculations due to the shortcomings of the local density approximation (LDA). However, the interpretation in terms of an embedded Peierls instability was supported by calculations for MoO 2 and NbO 2 [5, 6]. The …